1. Field of the Invention
The invention relates to an internal combustion engine exhaust gas purification apparatus and a method for controlling the same apparatus.
2. Description of the Related Art
Exhaust gas purification apparatuses have been proposed which incorporate a high-temperature activation NOx catalyst unit and a low-temperature activation NOx catalyst unit both provided in an exhaust gas passage to remove NOx. For example, Japanese Patent Application Publication No. 2004-218475 (JP-A-2004-218475) describes an exhaust gas purification apparatus in which a NOx selective reduction catalyst unit is provided upstream of a NOx storage-reduction catalyst unit and urea is injected into the exhaust gas passage. Thus configured, this exhaust gas purification apparatus is capable of removing NOx both at a low temperature and at a high temperature. More specifically, in the exhaust gas purification apparatus described in this publication, when the temperature of the exhaust gas at the inlet of the NOx selective reduction catalyst unit is low, reducing agent (e.g., urea) is not supplied and NOx is removed at the NOx storage-reduction catalyst unit. On the other hand, when the temperature of the exhaust gas at the inlet of the NOx selective catalyst unit is high, reducing agent is supplied so that NOx is removed at the NOx selective reduction catalyst unit. For example, if urea is supplied into exhaust gas as reducing agent, ammonia (NH3) is produced from the urea through its hydrolysis. This ammonia reacts with NOx at NOx selective reduction catalyst unit, whereby NOx is removed.
Further, Japanese Patent Application Publication No. 2006-274986 (JP-A-2006-274986) describes an exhaust gas purification apparatus in which a NOx selective reduction catalyst unit is provided downstream of a NOx storage-reduction catalyst unit, and fuel (HC) supplying means provided upstream of the NOx storage-reduction catalyst and urea supplying means provided upstream of the NOx selective reduction catalyst are controlled based on the temperatures of the catalyst units.
Meanwhile, when ammonia is produced from urea, if the reaction for producing ammonia is insufficient, cyanic acid (HNCO) unconvertible into ammonia may be produced. While such cyanic acid is being produced, the production amount of ammonia inevitably decreases, and thus the NOx removal rate decreases accordingly. However, JP-A-2004-218475 and JP-A-2006-274986 do not address this issue at all.